Exchange crown ether

Early examples of enantioselective extractions are the resolution of a-aminoalco-hol salts, such as norephedrine, with lipophilic anions (hexafluorophosphate ion) [184-186] by partition between aqueous and lipophilic phases containing esters of tartaric acid [184-188]. Alkyl derivatives of proline and hydroxyproline with cupric ions showed chiral discrimination abilities for the resolution of neutral amino acid enantiomers in n-butanol/water systems [121, 178, 189-192]. On the other hand, chiral crown ethers are classical selectors utilized for enantioseparations, due to their interesting recognition abilities [171, 178]. However, the large number of steps often required for their synthesis [182] and, consequently, their cost as well as their limited loadability makes them not very suitable for preparative purposes. Examples of ligand-exchange [193] or anion-exchange selectors [183] able to discriminate amino acid derivatives have also been described. 

Inspired by the separation ability of cyclic selectors such as cyclodextrins and crown ethers, Malouk s group studied the synthesis of chiral cyclophanes and their intercalation by cation exchange into a lamellar solid acid, a-zirconium phosphate aiming at the preparation of separation media based on solid inorganic-organic conjugates for simple single-plate batch enantioseparations [77-80]. 

It is obvious that the primary amines formed in this reaction will be uncontaminated by secondary or tertiary amines (unlike 10-44). The reaction is usually rather slow but can be conveniently speeded by the use of a dipolar aprotic solvent such as DMF or with a crown ether. Hydrolysis of the phthalimide, whether acid or base catalyzed (acid catalysis is used far more frequently), is also usually very slow, and better procedures are generally used. A common one is the Ing-Manske procedure,in which the phthalimide is heated with hydrazine in an exchange... 

Another material based on the crown ether extractant 4,4 (5 )-bis(t-butyl-cyclohexano)-18 crown-6, marketed under the name Sr-Spec, is useful for separations involving divalent cations including Pb, Ba, and Ra (Horwitz et al. 1991). For Ra analysis by TIMS, Ra-Ba separations are required because the presence of Ba drastically decreases the ionization efficiency of fg Ra samples from 10% to <1%. This material has been widely used for separations of Ra from Ba (e.g., Chabaux et al. 1994 Lundstrom et al. 1998 Rihs et al. 2000 Joannon and Pin 2001 Pietruszka et al. 2002) and is a complement or alternative to cation exchange separations for EDTA complexes of these elements (Volpe et al. 1991 Cohen and O Nions 1991). Sr-Spec material would also be useful for °Pb analysis, since Pb has a greater distribution coefficient than Sr with this extractant. 

Two new chromogenic crown ether derivatives based on an identical design principle have been synthesized. The lipophilic KBC-002 shown in Figure 23 is a useful new chromo- ionophor for the highly selective calcium determination with cation exchange type optode53. 

Finally, crown ethers have also been shown to produce marked changes in the stereochemistry of ElcB-reactions of methoxyacenaphthenes [189] promoted by KOt-Bu/t-BuOH (Hunter and Shearing, 1971). Addition of dicyclohexyl-18-crown-6 increases the ratio of the rates of exchange and elimination from 1.3 to 3.2, while for both exchange and elimination the cis/trans ratio decreases. 

De Jong et al. (1976c, 1977) have studied the rate of decomplexation of t-BuNHjPFj (AM) complexes of crown ethers. The exchange broadening of the t-butyl nmr signal was used to determine the rate of cation exchange between a kinetically stable (X.AM) and a kinetically unstable (Y.AM)... 

For crown ethers in which the H-nmr probes Ha and Hb are diastereotopic in the free crown ether too, the cation exchange process (50) becomes more complicated. For 1 1 complexes the measured free energy of activation also involves conformational inversion components (AG = AG + AG ), whereas exchange between free and complexed crown ether (2 1 ratio of crown ether to salt) only involves the free energy of activation for decomplexation (AG ). Bradshaw et al. (1979b) have recently reported AG -values for cation exchange in RNH3X complexes of crown ethers [257]. The AGf term was estimated to be 0.7, 0.5, and 1.2 kcal mol-1 for compounds [257 n = 1] with... 

Sutherland and co-workers have studied the cation-exchange processes in alkylammonium complexes of crown ethers [258]—[262], Depending on the position of the exchanging H-nmr sites, the measured zlG -values may refer to three different types of cation exchange (0 cation exchange between free and... 

Free energies of activation (AG in kcal mol-1 atTc) for exchange of RNH3X in diaza-crown ether complexes in CD2C12... 

Eujii T, Suzuki D, Gunjii K, Watanabe K, Moriyama H, Nishizawa K (2002) Nuclear field shift effect in the isotope exchange reaction of chromium(III) using a crown ether. J Phys Chem A 106 6911-6914 Gale JD (2001) Simulating the crystal structures and properties of ionic materials from interatomic potentials. Rev Mineral Geochem 42 37-62... 

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